1. Field of the Invention
The present invention relates to electrical motor drives and, in particular, to electric motors driven by switched converters which convert a dc potential to one or more phases of pulsed current to drive the motor. The motor can be, for example, a brushless dc motor having Hall sensors to control the commutation.
This invention further relates to a power steering device that generates auxiliary steering power for driving the steering mechanism of a vehicle by means of the oil pressure that is generated by a pump which is driven by electric power.
The invention further relates to reducing EMI (electromagnetic interference) in a motor drive for controlling an electric motor.
2. Technology According To Prior Art
FIG. 1 shows a typical three phase motor drive from a dc bus. The motor may be a brushless DC motor having a permanent magnet rotor and a stator comprising stator coils fed with switched pulsed phase drive signals. The dc bus voltage is provided to an inverter 100 comprising three half bridges comprising transistors (e.g., MOSFETs, IGBTs, bipolar devices) gated by signals AH, AL, BH, BL and CH, CL. The high and low side devices are each connected in series across the bus and the output of each device comprises one of the three phases, U, V and W. Each of the switching devices is controlled by a controller 200, which receives Hall signals controlling the commutation times from the electric motor 300. The gate drive signals AH, AL, BH, BL and CH, CL are provided to the respective switches of the inverter 100.
In a typical motor drive, shown, for example in FIG. 2, a Hall signal is provided from the motor for each phase, one of which is shown. Only one of each of the gate drive high and low signals is shown. In a typical application, the Hall signals provide a signal for controlling the switching of the switches in the inverter and thus the motor commutation. A typical motor drive is shown in FIG. 2 having a 120° conduction angle. As shown, the gate drives can be pulse width modulated (PWM) as shown by the low gate drive signal in FIG. 2. The gate drive signal switch events occur when the Hall transitions occur and any phase advance of the gate drive signal is determined solely by the physical placement of the position of the Hall effect sensors in the motor. The conduction angle is forced to be 120° or 180°. The effective voltage at the outputs of the half bridges is controlled by varying the duty cycle of the PWM. The pulse width modulation may be done on the low side or the high side or on both the high side and the low side. In FIG. 2, only one phase is shown. The other two phases are shifted by 120°.
FIG. 3 shows another example of a typical motor drive having 180° conduction angle. Similarly, the high or low side signals can be pulse width modulated or both can be pulse width modulated.
In the past, if a phase advance of a gate drive signal was desired, this was obtained solely by the physical placement of the position Hall effect sensors in the motor. That is, to obtain a phase advance, the position of the sensor in the motor would be moved forward by a certain number of degrees depending upon the desired phase advance. This phase advance is fixed and not electrically variable.
An object of the present invention is to provide a means for achieving a variable phase advance and/or conduction angle requiring no mechanical changes to the motor to obtain phase advance and change the conduction angle, thereby resulting in improved motor control.
It is a further object of the invention to provide an improved electric power steering system for a vehicle.
A power steering device that assists the operation of the steering wheel of a vehicle by supplying operating oil from the oil pump to the power cylinder that is joined to the steering mechanism has been known. The oil pump is driven by an electric motor, with the auxiliary steering power which is in conformity with the speed of the motor rotation being generated by a power cylinder.
Into the steering shaft, a torsion bar that generates torsion which is in conformity with the direction and size of the steering torque which has been provided by the steering wheel and an oil pressure control valve which changes its opening size in conformity with the direction and the size of the torsion of the torsion bar are incorporated. This oil pressure control valve is provided in the oil pressure system between the oil pump and the power cylinder and it causes an auxiliary steering power which is in conformity with the steering torque to be generated from the power cylinder.
The drive control of the electric power motor is carried out on the basis of the steering angular speed of the steering wheel. The steering angle speed is obtained on the basis of the output of the steering angle sensor that has been provided in connection with the steering wheel, and the target rotary speed of the electric power motor is set based on this steering angle rate. Voltage is supplied to the electric motor in such that this target rotary speed may be achieved.
As the electric motor, a triple-phase brushless motor is ordinarily used. The triple-phase brushless motor comprises a stator which has field coils for the U phase, the V phase and the W phase, a rotor with a fixed permanent magnet that receives the repulsive magnetic field from the field coils and Hall sensors for detecting the rotation position of this rotor. Three Hall sensors are provided at an interval of 120 degrees as an electric angle in conformity with the U phase, the V phase and the W phase.
The triple-phase brushless motor is driven in accordance with the conventional 120 degree power system in the ordinary case. This 120 degree power system is shown in FIG. 13. The Hall signals that are outputted by the Hall sensors of the U phase, the V phase and the W phase deviate from each other by 120 degrees in phase. The electrical power is passed during a period corresponding to an electric angle of 120 degrees to the U phase, the V phase and the W phase in turn so as to synchronize with the Hall signals of the U phase, the V phase and the W phase. It becomes possible to change the rotary speed of the brushless motor by the PWM (pulse width modulation) control of the supply of the drive current to each field coil during the electricity-conducting period of 120 degrees.
FIG. 14 shows the relationship between the rotary speed of the rotor and the output torque in the triple-phase brushless motor. As is shown in FIG. 14, it is known that the output torque decreases along with an increase in the rotary speed. As can be understood from the formula relating to the motor as shown in (1) below, if the rotary speed of the motor (ω) increases, the electric current I that flows to the motor decreases along with an increase in the motor-generated induced voltage kω, also known as the back emf, with a result that the output torque that is proportional to the electric current I becomes smaller.V=IR+L di/dt+kω  (1)where L=motor inductance di/dt=rate of change of current and V indicates the voltage impressed to the motor, I is the electric current that flows to the motor, R is the electric resistance of the motor, K is a constant and ω indicates the speed of rotation of the motor.
A problem in conventional electronic motor drives, particularly those employing pulse width modulation (PWM) of the switches of the drive inverter, is that the high frequency switching of the switches results in considerable emission of electromagnetic interference (EMI), which can interfere with the operation of other equipment, such as audio and video equipment, radios and computers. This is particularly troublesome in a number of applications, such as in an automobile, where the EMI may interfere with radio equipment and the automobile engine management system, as well as other systems external to the automobile. In addition, regulations have been imposed by governmental bodies that require EMI emissions to be below present levels, usually dependent on frequency.